1. Field of the Invention
The present invention relates to an automatic modulation control in a transmitter. More specifically, the present invention relates to an improvement in an automatic modulation control in a transmitter which is switchable as a receiver as in case of a transceiver.
2. Description of the Prior Art
An automatic modulation control is provided in a transmitter for the purpose of preventing overmodulation and preventing energy distribution from being widespread in terms of the occupied bandwidth. Necessity of an automatic modulation control is also ruled by Federal Communications Commission of the United States.
The major important requirements for an automatic modulation control are listed in the following.
(1) A stabilized feedback operation without overmodulation in a necessary frequency band.
(2) No variation in the modulation percentage in spite of variation of the temperature and the source voltage.
(3) A quick response rate to variation of the modulation percentage.
(4) A predetermined modulation factor performed throughout a wider range of the input, i.e. a wide dynamic range for allowing for control of modulation.
(5) No frequency dependent characteristic with respect to an input voltage for operation of an automatic modulation control.
(6) Inexpensive implementation and simple adjustment.
FIG. 1A shows a block diagram of a typical conventional transceiver employing an automatic modulation control in a transmitter mode. The transceiver shown comprises a receiver portion RC for receiving a transmitted wave signal to convert the same into an audible sound, and a transmitter portion TM for converting an audible sound into a transmitting wave to transmit the same. Such transceiver may comprise a frequency signal generator 120, which may be implemented by a frequency synthesizer employing a phase-locked loop, for providing a local oscillation frequency output and a carrier frequency output to the receiver portion RC and the transmitter portion TM, respectively, and a transmitter/receiver selection switches SW1, SW2, SW3 and SW4, which are coupled in a ganged fashion to a press-to-talk switch for selectively switching a transmitting mode or receiving mode of the transceiver.
The receiver portion RC comprises an antenna 11 for receiving a transmitted wave, a high frequency amplifier 111 for amplifying the received wave signal, a first mixer 112 for mixing the high frequency output from the amplifier 111 with a first local oscillation frequency output from the frequency signal generator 120 for providing a first intermediate frequency output, a second mixer 113 for mixing the first intermediate frequency output from the first mixer 112 with a second local oscillation frequency output from the frequency signal generator 120 for providing a second intermediate frequency output, an intermediate frequency amplifier 114 for amplifying the second intermediate frequency output from the second mixer 113, a detector 115 for detecting the intermediate frequency output from the amplifier 114 for providing an audio frequency amplifier, a power amplifier 3 for amplifying the audio frequency output from the detector 115, and a loudspeaker SP for transducing the audio frequency output from the amplifier 3 into a sound output. Preferably, a squelch control 116 is coupled to the detector 115 which is selectively operable in the receiver mode by virtue of selection of the transmitter/receiver selection switch SW1 at a predetermined level as preset by a variable resistor 117. An output transformer 4 is interposed between the power amplifier 3 and the loud speaker SP. A variable resistor 118 is coupled to the detector 115 for the purpose of volume control of the audio frequency output.
The transmitter portion TM comprises a microphone 1 for converting a sound into an audio electrical signal, a preamplifier 2 for amplifying the audio electrical signal, an audio amplifier shared by the above described power amplifier 3 for power amplifying the audio electrical signal from the microphone 1, a modulator including a buffer amplifier 122, a driver amplifier 123 and a final stage amplifier 10 for modulating a carrier signal of the carrier frequency output from the frequency signal generator 120 with the amplified audio signal for providing a modulated signal, a low pass filter or bandpass filter 9 for selectively withdrawing the modulated signal from the modulator, and the antenna 11 serving as a transmitting antenna for transmitting the high frequency output of the modulated signal from the filter 9. The transmitter portion TM further comprises an automatic modulation control 12 which is coupled from the transformer 4 to the input of the preamplifier 2 as a feedback loop. The transmitter/receiver selection switch SW2 is coupled to the frequency signal generator 120 for the purpose of switching the operation of the frequency signal generator 120 among the transmitter mode and the receiver mode. The transmitter/receiver selection switch SW3 is coupled to the preamplifier 2 for the purpose of disabling the transmission path between the microphone 1 and the power amplifier 3 in the receiver mode. The transmitter/receiver selection switch SW4 is interposed between the loudspeaker SP and ground, or electrical common, for the purpose of disabling the loudspeaker SP in the transmitter mode. For the purpose of selection of channels, a channel selector 121 is coupled to the frequency signal generator 120. For the purpose of displaying the selected channel, a channel indicator 119 is coupled to the channel selector 121 and the frequency signal generator 120.
FIG. 1 shows a schematic diagram of a major portion of the transmitter portion TM in the FIG. 1A transceiver. It is pointed out that FIG. 1 specifically shows a schematic diagram, partially in a block form, of the microphone 1, a preamplifier interposed between the microphone 1 and the power amplifier 3, the transformer 4, the speaker SP, the final amplifier 10, the low pass filter 9, and the automatic modulation control 12.
The primary winding of the transformer 4 is connected to a positive voltage source VDD1 at one end and is coupled through a diode D1 at the other end to a junction 8. The junction 8 is grounded through a decoupling capacitor C1 to the ground 6. The junction 8 is also connected through a load coil L to the collector of a transistor Q2 which constitutes the final amplifier 10. The base electrode of the transistor Q2 is coupled from the driver amplifier 123. The junction between the load coil L and the collector electrode of the transistor Q2 is connected through a line 7 to the low pass filter 9 to supply the high frequency output of the modulated signal to the low pass filter 9. The decoupling capacitor C1 serves to render the impedance at the junction 8 at a low impedance with respect to the carrier high frequency and at a high impedance with respect to an audio low frequency of several KHz. The load coil L serves also as a portion of the above described low pass filter 9. The transistor Q2 is connected as a C class power amplifier. Therefore, if and when the high frequency input is applied in the transmitter mode to the base electrode of the transistor Q2 the transistor Q2 operates, partially as on/off operable, by virtue of being class C amplifier and an on/off output is developed at the collector electrode of the transistor Q2, which is smoothed by the load coil L and the decoupling capacitor C1 to provide a direct current component at the junction 8. However, since the above described direct current component developed at the junction 8 is lower than the voltage of the direct current source +VDD1, the diode D1 is rendered conductive in the transmitter mode. As a result, the modulation signal as amplified by the power amplifier 3 is allowed to pass through the diode D1 to the final stage amplifier 10. On the other hand, if and when no high frequency input is applied in the receiver mode to the base electrode of the transistor Q2, the transistor Q2 is rendered non-conductive and accordingly the diode D1 is rendered non-conductive. As a result, the modulation signal obtained from the transformer 4 is not allowed to pass through the diode D1 to the final amplifier 10.
The junction 8 is coupled through a line 13 to the automatic modulation control 12. More specifically, the automatic modulation control 12 shown comprises an RC circuit including a capacitor C2 and a variable resistor VR1 for blocking a direct current component, a rectifying circuit including diodes D2 and D3 for rectifying in a voltage doubler manner an alternating current component obtained through the above described RC circuit, a smoothing circuit including a capacitor C3 and a resistor R1, a zener diode D4 for threshold detecting the smoothed output, a further smoothing circuit and an amplifier including a transistor Q3. In operation, a direct current component of the output obtainable at the junction 8 is blocked by the above described RC circuit including the capacitor C2 and only an alternating current component thus obtained is rectified in a voltage doubler manner by the diodes D2 and D3. The rectified output is smoothed by the capacitor C3 and the resistor R1 to provide a smoothed output at the junction 15. If and when the smoothed output exceeds a predetermined threshold level determined by the zener diode D4, the zener diode D4 becomes conductive to provide an output at the anode electrode of the zener diode D4. The threshold detected output is then smoothed by the smoothing circuit and is applied to the base electrode of the transistor Q3, thereby to decrease the impedance between the collector and the emitter electrodes of the transistor Q3. The decrease of the impedance through the transistor Q3 serves to partially shunt the microphone input signal, thereby to reduce the input to the preamplifier 2. The variable resistor VR1 is used to adjust a suitable level for preventing overmodulation. The preamplifier 2 is implemented by a transistor Q1, which is connected between a positive voltage source +VDD3 and the ground.
From the foregoing description, it will be appreciated that according to the automatic modulation control 12 shown the output of the modulation signal amplifier including the preamplifier 2 and the power amplifier 3 obtainable from the junction 8 is applied through the capacitor C2 for the purpose of blocking a direct current component, thereby to provide only an alternating current component, which is then rectified and is smoothed for the purpose of threshold detection. In other words, according to the automatic modulation control 12 shown an average value of the modulation signal is evaluated by way of a rectified and smoothed output, which is then threshold detected at a predetermined threshold level for the purpose of controlling the input level of the preamplifier of the modulation signal. However, it has been observed that such automatic modulation control involves several shortcomings to be described subsequently. Hence, there is room for improvement in the prior art automatic modulation control. Several typical disadvantages encountered by the prior art automatic modulation control as shown will be listed in the following.
(1) Since a signal transmission path in the automatic modulation control comprises the blocking capacitor C2 in series and further comprises other capacitors C3 and C4, while the same includes only the single transistor Q3 as an active device, the feedback gain is not sufficiently large, with the result that a feedback operation is unstable depending on the frequencies and oscillation is caused in the worst situation.
(2) Since the zener diode D4 and the diodes D2 and D3 have a temperature dependent characteristic, modulation is subject to variation of the ambient temperature.
(3) Assuming that the source voltage fluctuates, for example, the source voltage becomes lower, the peak voltage of the carrier wave becomes lower, while the amplitude of the modulation signal is kept unchanged, which could cause overmodulation because of an increase in the modulation factor. Conversely, assuming that the source voltage becomes higher, then the voltage of the carrier wave accordingly increases, while the amplitude of the modulation signal is kept constant, with the result of a decrease of modulation.
(4) Since only the single transistor Q3 is employed as an active device in the automatic modulation control, the gain of the automatic modulation control is small and hence the dynamic range allowing for control of modulation is narrow.
(5) For the above described reasons, the response rate to variation of the modulation factor is slow.
(6) Since the automatic modulation control shown comprises three capacitors having a frequency dependent characteristic, the bias voltage of the transistor Q3 varies in the range where the modulation signal is so large that the automatic modulation control becomes operable, with the result that the operation of the automatic modulation control exhibits a frequency dependent characteristic, i.e. the effect of modulation control is different dependent on the frequencies.
The present invention is aimed to eliminate the above described shortcomings as much as possible.